A half-tone reproduction of an original image, produced on an appropriate receiving medium, has a dot pattern, i.e., consists of a plurality of separate dots having greater areas (sizes) on darker portions of the image than on its light portions. A typical method of making a half-toe reproduction of an image includes generating signals corresponding to the reflectivities of separate unit areas of the image to be reproduced and forming dots in the receiving medium unit areas corresponding to the unit areas of the image. The area of a dot in each unit area of the receiving medium depends on the reflectivity of the image unit area corresponding to this unit area of the receiving medium. When a positive image is formed, the dot area is the greater, with the smaller of the reflectivity of the corresponding image unit area. When a negative image is formed, the dot area is the greater, with the greater of the reflectivity of the corresponding image unit area. Each dot usually lies in the center of a corresponding unit area of the receiving medium.
When half-tone dots are formed in unit areas corresponding to image unit areas intersected by a continuous contour, i.e., by a boundary between a relatively very dark portion and a relatively very light portion of the image, the relative position of the contour and a unit area affects only the area of a corresponding dot (which area is determined by the average reflectivity of the unit area divided by the contour into a light portion and a dark portion). In such a case a considerable part of the dot, and even its greater part or the whole dot, may be formed on that side of the contour which should be light. As a result, the contour becomes slurred and is perceived visually as having a stepped shape. Deterioration in the perception of a contour becomes especially noticeable when the contour passes at a small angle to the line along which the half-tone dots are disposed. When small elements of an image are reproduced, this situation leads to a noticeable distortion of their shape or to their blurring, and, in the case of very small elements, even to their disappearance during reproduction. All this results in deterioration of the quality of the obtained image.
Known is a method of making a half-tone reproduction of an image (U.S. Pat. No. 3,904,816), according to which the unit areas on the receiving medium are formed in such a way that the reflectivity of a receiving medium unit area is determined not only by the reflectivity of the respective unit area of the image to be reproduced, but also by the reflectivities of the adjacent unit areas adjoined it on different sides. According to this method, the signal corresponding to the reflectivity of a given image unit area is corrected by a value determined by the difference between the reflectivity of this unit area and the arythmetic mean of the reflectivities of this unit area and the adjacent unit areas. This method is known as "electronic unsharp masking".
With such a method, the image near the boundary dividing its dark and light portions becomes somewhat darker on the dark side and somewhat lighter on the light side as if "underlying" the image contour. In some cases this improves visual perception of the contour and creates an illusion of a sharper reproduction of the contour. However, the stepped shape of the contour is not actually smoothed and may become even more noticeable, i.e., the sharpness of image reproduction is not really improved. For the same reason, such a method does not improve reproduction of small image elements.
Known is a method (U.S. Pat. No. 3,498,108) including determining the reflectivity of each of the four portions making up an image unit area corresponding to a half-tone dot of the image reproduced on the receiving medium. With this method, each half-tone dot is made up of four parts adjoining one another and the center of the respective unit area, each part lying within that portion of the receiving medium unit area which corresponds to the respective portion of the image unit area and having an area determined by the reflectivity of this portion. A half-tone dot in a unit area intersected by an image contour is deformed in such a way that the area of the dot part located on the dark side of the contour becomes greater, while the area of the dot part located on the light side of the contour becomes smaller, Some, more or less considerable, part of the dot in this case always remains in the central area of the respective unit area.
Such deformation of a half-tone dot towards a dark side of the image allows the stepped shape of the contour to be smoothed to some extent and the contour itself to be made somewhat sharper. Such a method makes it also possible to reduce to some extent distortions in the shapes of small elements and their blurring. However, since the dot parts are grouped together around the center of the respective unit area, a considerable part of the dot, and even its greater part or the whole dot, may in this case also be found on that side of the contour which should be light. As pointed out above, this adversely affects visual perception of the contours and small elements of the reproduced image and thus the quality of its reproduction.
Besides, the need to determine four reflectivity values for each image unit area leads to a corresponding increase in the amount of the data according to which the image is reproduced, and hence in the time required for processing and transmitting this data.
Known is a method of making a half-tone reproduction of an image (GB, A, 2105114), according to which each of the receiving medium unit areas corresponding to the image unit areas the reflectivities of which are to be determined is divided into a plurality of smaller elements forming a rectangular grid. The half-tone dots are formed according to the results obtained by comparing the reflectivities of the image unit areas with the values of a certain predetermined half-tone function which is a two-dimensional periodic function of the mutually orthogonal coordinates of said elements. Such a method includes generating signals corresponding to the reflectivities of the image unit areas and signals corresponding to the half-tone function. The signal corresponding to the reflectivity of an image unit area is compared with the signals corresponding to the half-tone function values determined by the coordinates of the elements of the receiving medium unit area corresponding to this image unit area. Each of the elements of the receiving medium unit areas is formed dark or light depending on the difference between the half-tone function value determined by the coordinates of this element and the reflectivity of the image unit area corresponding to the receiving medium unit area incorporating this element. If a positive image is to be produced on the receiving medium, an element is formed light when the reflectivity value is greater than the half-tone function value, and dark when the reflectivity value is smaller than the half-tone function value. If a negative image is to be produced on the receiving medium, an element is formed light when the reflectivity value is smaller than the half-tone function value and dark when the reflectivity value is greater than the half-tone function value.
According to this method, each dot is constituted by a plurality of dark elements formed within one spatial period of the half-tone function, where its values differ from a corresponding reflectivity value (or from corresponding reflectivity values if the elements of the half-tone dots are located in receiving medium unit areas corresponding to different image unit areas) in a predetermined sense, i.e., in a positive sense when a positive image is to be produced and in a negative sense if a negative image is to be produced. The number of such dark elements, and hence the dot area, will therefore depend on the reflectivity of the corresponding image unit area.
If such a method is used to form half-tone dots on unit areas intersected by an image contour, the disposition of the contour will effect only the area of each such dot, which area will be determined by the reflectivities of the image unit areas corresponding to the receiving medium unit areas incorporating the elements constituting the dot. As a result, a considerable part of the dot, and even its greater part or the whole dot, may be formed on that side of the contour which should be light. This adversely affects reproduction of contours and small elements of an image and thus the quality of the reproduced image.
To make a half-tone reproduction of an image by comparing the reflectivities of the image unit areas with the values of a half-tone function, it is known to use a device (GB, A, 2105144) comprising a video signal source for sequentially producing reflectivity values of image unit areas. The device further comprises a light source, a means for sequentially directing the beam of the light source to elements of the receiving medium unit areas corresponding to image unit areas, a controlled interrupter for interrupting the beam of the light source, a timing unit and a half-tone function generating unit for generating values of a half-tone function which is a periodic function of the coordinates of the receiving medium unit area element. A coordinate generating unit for generating coordinates of the elements of the receiving medium unit areas is connected between the timing unit and the half-tone generating unit. The control input of the interrupter is connected to the output of a comparison unit having its one input connected to the video signal source and its other input connected to the output of the half-tone function generating unit. The coordinate generating unit produces corresponding addresses according to which the half-tone function values stored in the half-tone function generating unit are extracted therefrom. These values are compared in the comparison unit with corresponding reflectivity values coming from the video signal source.
This apparatus has the same disadvantages as the method which it implements.